The present invention relates to a fixed point detecting device and more particularly, to a fixed point detecting device using detection of light diffracted by holographic diffraction gratings, which is suitable for detection of a dislocation of a substrate upon multiple exposure of an integrated circuit, and detection of the origin of an encoder, etc.
As for an X-ray exposure drawing device for manufacturing integrated circuits and a length measuring device to be used for accurate machining, a reference point or an origin is established to measure an exact position or distance. A fixed point detecting device is used for establishing such reference point or origin.
One conventional fixed point detecting device is disclosed in JP-A 61-158501. This device is constructed to detect a fixed point through a mark, and includes a laser generator, a position sensor, etc. A laser beam out of the laser generator is diffracted by diffraction gratings of the mark, and first-order diffracted light is detected by the position sensor. The position sensor serves to determine an angle of diffraction at which the intensity of diffracted light is maximum. A value of the angle of diffraction at which the intensity of diffracted light is maximum is varied when a laser beam spot passes across a boundary of two portions of the mark, i.e. before and behind the boundary of the two portions of the mark. The fixed point is detected by such variation in the angle of diffraction.
Another conventional fixed point detecting device is a variant of the above device. This variant uses two transmission and volume-type holographic diffraction gratings or holograms and two photo detectors. As for the transmission and volume-type holographic diffraction gratings, diffracted lights goes out on the side opposite to incident light with respect to the holographic diffraction gratings. Therefore, the photo detectors are disposed on the side opposite to the laser generator with respect to the holographic diffraction gratings. The holographic diffraction gratings have grating intervals or grating pitches different from each other. Zero-order diffracted lights, positive first-order diffracted lights, negative first-order diffracted lights, and high-order diffracted lights are obtained by the diffraction gratings. Among them, the photo detectors detect positive first-order diffracted light.
Another conventional fixed point detecting device is disclosed in JP-A 4-324318. This device includes a stationary portion and a movable portion which is movable in the direction of measurement, the stationary portion having an optical system and a detecting system, and the movable portion having a substrate and two volume-type holographic diffraction gratings or holograms disposed thereon. The two holograms are disposed on the substrate on an upper side thereof to be adjacent to each other. The two holograms are constructed symmetrically with respect to a center plane. That is, angles of inclination of distributed planes of the holograms are symmetrically and continuously varied on both sides of the center plane, and grating intervals or grating pitch thereof are symmetrically and continuously varied on both sides of the center plane. The two holograms are disposed so that points at which the diffraction efficiencies become maximum are different from each other in the direction of measurement.
When the movable portion is moved relative to the stationary portion, i.e., with respect to light receivers and a light source which are stationary, light diffracted by the first hologram is detected by the first light receiver, whereas light diffracted by the second hologram is detected by the second light receiver. As for the two holograms, the points at which the diffraction efficiencies become maximum are different from each other, so that a peak position of a luminous intensity curve of diffracted light detected by the first light receiver is different from a peak position of a luminous intensity curve of diffracted light detected by the second light receiver. That is, there exists a point at which the two luminous intensities are equal to each other. Such a point is a fixed point obtained by this fixed point detecting device.
However, the above conventional fixed point detecting devices have the following drawbacks.
As for the first prior art reference, the position sensor serves as a light receiving device. The position sensor is constructed to detect an angle of diffraction at which the intensity of diffracted light is maximum, resulting in low resolving power. Moreover, a position sensor is expensive which allows accurate detection of the angle of diffraction.
As for the second prior art reference, the two holographic diffraction gratings have grating intervals or grating pitches different from each other, and the positive first-order diffracted light and the negative first-order diffracted light go out on both sides with respect to the direction of incident lights. Therefore, in order to detect the two positive first-order diffracted light, the two photo detectors should be disposed adjacently. Moreover, the two positive first-order diffracted light should be separated completely for detection through the photo detectors. This can be obtained by simply enlarging a difference between the grating interval of the first hologram and the grating interval of the second hologram. However, when enlarging a difference between the grating intervals of the two holograms, an error is increased with a variation in a wavelength of light out of the light source.
As for the third prior art reference, since the two holograms are constructed symmetrically with respect to the center plane, the fixed point is not changed even with a variation in the wavelength of light out of the light source. However, the two light receivers should be disposed accurately with respect to the two holograms. If not, the resolving power is lowered.
It is, therefore, an object of the present invention to provide a fixed point detecting device which enables accurate determination of a fixed point.